This invention relates generally to fatigue simulators, and more particularly, to a device which is capable of mechanically simulating the cyclic stress or fatigue which is undergone by nickel battery electrodes or more specifically, electrode plaque during battery operation.
Currently, more and more emphasis is being placed upon to the development of high performance, compact, lightweight miniaturized power sources. Such power sources generally are in the form of high quality, nickel-cadmium, nickel-zinc, and nickel-hydrogen cells or batteries. Such batteries have many applications, although their primary application is in use as an aircraft emergency power source and/or spacecraft power source.
Historically, however, there has been little interest in the mechanical characteristics of the sintered battery electrode substrate material, more commonly referred to as electrode plaque or simply by the term plaque. This substrate (plaque) has been characterized primarily in terms of its chemical compatibility, porosity, current carrying capability, and surface area. Plaque has commonly been viewed as merely an immobile and inert "container" for the active chemical components or chemically active electrode material.
Recently attention has been given to the fatique characteristics of the electrode plaque as fatigue has been shown to result in long term capacity degradation of nickel electrodes. More specifically, a primary irreversible failure mechanism of nickel electrodes is the mechanical fatigue of the plaque. The fatigue failure is a result of cyclic mechanical stresses acting on the electrode plaque and which are directly related to the number of charge-discharge cycles of a battery. The implication of this conclusion is that an improved cycle life can be obtained from nickel electrodes by building a fatigue-resistant plaque. This is particularly important in the Ni-H.sub.2 battery in which the life-limiting component of the battery is the nickel electrode. Consequently, the plaque configuration and mechanical properties must be engineered to obtain maximum efficiency and life from the nickel electrode.
Currently available fatigue or stress testing equipment is inadequate for electrode plaque testing. For example, fatigue testing by bending which has yielded meaningful results leaves much to be desired since the precise location of the current collector grid within the plaque must be known before such results can be obtained. Consequently, it becomes necessary to select only appropriate test samples instead of random test samples, which, of course, is much more desirable.
It is therefore readily apparent that special equipment that can apply small and cyclic test loads to electrode plaque is required. It is essential in the production of more efficient batteries to provide a battery electrode stress or fatigue simulator which is capable of accurately reproducing the cyclic stresses incurred during battery operation.